Ceramic double layer based on zirconium oxide

ABSTRACT

At least two ply layers of ceramic on a substrate which is applied to protect a surface in a heated hot environment. Each of the outer and bottom layers including zirconium oxide and stabilizers of yttrium oxide in different respective proportions of the yttrium oxide; the outer layer has fully stabilized zirconium oxide and the bottom layer has partially stabilized zirconium oxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 35 U.S.C. §§371 national phase conversion of PCT/EP2012/065458, filed Aug. 7, 2012, which claims priority of European Patent Application No. 11185023.6, filed Oct. 13, 2011, the contents of which are incorporated by reference herein. The PCT International Application was published in the German language.

FIELD OF THE INVENTION

The invention relates to a two-ply ceramic layer system based on zirconium oxide.

TECHNICAL BACKGROUND

The use of zirconium oxide as a single layer on turbine blades or vanes is known.

It is similarly known to use a plurality of ceramic layers having an inner zirconium oxide layer and an outer oxide layer based on pyrochlore.

SUMMARY OF THE INVENTION

It is an object of the invention to further simplify and to improve existing ceramic coatings.

The two ply ceramic layer system of the invention includes at least two ply layers of ceramic on a substrate for protecting a surface in a heated hot environment. Each of the outer and bottom layers includes zirconium oxide and stabilizers of yttrium oxide in different respective proportions of the yttrium oxide. The outer layer has fully stabilized zirconium oxide and the bottom layer has partially stabilized zirconium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a layer system,

FIG. 2 shows a list of superalloys,

FIG. 3 shows a turbine blade or vane,

FIG. 4 shows a combustion chamber, and

FIG. 5 shows a gas turbine.

DESCRIPTION OF EMBODIMENTS

The description and the Figures represent only exemplary embodiments of the invention.

FIG. 1 shows an example of a layer system 1 according to the invention.

The layer system 1 comprises a substrate 4.

This is preferably metallic and comprises a nickel-based or cobalt-based superalloy. In particular, use is made of superalloys as per FIG. 2.

A metallic bonding and corrosion-resistant layer 7 is optionally present on the substrate 4. These can be aluminide layers in various variations or preferably MCrAlX layers (X optionally=Y, Re, Si, Ta, Fe, . . . ).

An aluminum oxide layer which contributes to oxidation protection (not shown) is formed on the MCrAlX layer 7 or on the substrate 4 during operation (TGO) or preferably beforehand.

The preferably two-ply ceramic layer 15 in each case based on zirconium oxide, i.e. the proportion of zirconium oxide is at least 50% by weight, in particular at least 60% by weight, is present on the metallic bonding layer 7 or the substrate 4.

The ceramic layers 10, 13 have different properties.

The bottom ceramic layer 10 based on zirconium oxide is preferably partially stabilized.

The partial stabilization can preferably be effected by yttrium oxide with known proportions, preferably between 4% by weight and 12% by weight.

Other known stabilizers—alone or in addition—corresponding to the known additions for partial stabilization are likewise possible, for example magnesium oxide and/or other oxides.

The outer ceramic layer 13 is preferably a fully stabilized zirconium oxide layer.

The full stabilization is preferably likewise achieved by yttrium oxide in corresponding higher proportions, preferably between 20% by weight and 50% by weight, in particular between 30% and 40%. Other known stabilizers—alone or in addition—corresponding to the known additions for partial stabilization are likewise possible, for example magnesium oxide or other oxides.

This ceramic double layer system has the advantage that very high operating temperatures are achievable through the fully stabilized system of the outer layer 13, since the C phase in this system is stable throughout the temperature range. Therefore, no phase transitions arise in the top layer 13.

A good bond between the ceramic layers 10, 13 is provided as a result of the lower temperature at the inner ceramic layer 10 and the similar chemical systems of the layers 10, 13. Similarly, the high proportions of the stabilizer, in particular yttrium, make it possible to contain the losses thereof and to keep the system in the C phase region stable.

The two layers 10, 13 preferably have a porosity of >16%, in particular of 16% to 24%.

The outer layer 13 is at least 20%, in particular at least 30%, thicker than the bottom layer 10. The layer thickness ratio (10/13) is preferably 1/3 to 2/3.

The bottom ceramic layer 10 preferably has a thickness of up to 500 micrometers.

The second ceramic layer 13 preferably has a thickness of up to 1000 micrometers.

A layer system of this type can be used for components 120, 130, 155 (FIGS. 3 and 4) that are employed at high temperatures. These are in particular combustion chamber blocks 155, turbine blades or vanes 120, 130 for aircraft, gas turbines 100 (FIG. 5) and/or steam turbines.

FIG. 3 shows a perspective view of a rotor blade 120 or guide vane 130 of a turbomachine, which extends along a longitudinal axis 121.

The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor.

The blade or vane 120, 130 has, in succession along the longitudinal axis 121, a securing region 400, an adjoining blade or vane platform 403, a main blade or vane part 406 and a blade or vane tip 415.

As a guide vane 130, the vane 130 may have a further platform (not shown) at its vane tip 415.

A blade or vane root 183, which is used to secure the rotor blades 120, 130 to a shaft or a disk (not shown), is formed in the securing region 400.

The blade or vane root 183 is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible.

The blade or vane 120, 130 has a leading edge 409 and a trailing edge 412 for a medium which flows past the main blade or vane part 406.

In the case of conventional blades or vanes 120, 130, by way of example solid metallic materials, in particular superalloys, are used in all regions 400, 403, 406 of the blade or vane 120, 130.

Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blade or vane 120, 130 may in this case be produced by a casting process, also by means of directional solidification, by a forging process, by a milling process or combinations thereof.

Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally.

In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component.

Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures).

Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1.

The blades or vanes 120, 130 may likewise have coatings protecting against corrosion or oxidation, e.g. (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

The density is preferably 95% of the theoretical density.

A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer).

The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re.

It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists for example of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX.

The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).

Other coating processes are possible, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer.

Refurbishment means that after they have been used, protective layers may have to be removed from components 120, 130 (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component 120, 130 are also repaired. This is followed by recoating of the component 120, 130, after which the component 120, 130 can be reused.

The blade or vane 120, 130 may be hollow or solid in form. If the blade or vane 120, 130 is to be cooled, it is hollow and may also have film-cooling holes 418 (indicated by dashed lines).

FIG. 4 shows a combustion chamber 110 of a gas turbine. The combustion chamber 110 is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners 107, which generate flames 156 and are arranged circumferentially around an axis of rotation 102, open out into a common combustion chamber space 154. For this purpose, the combustion chamber 110 overall is of annular configuration positioned around the axis of rotation 102.

To achieve a relatively high efficiency, the combustion chamber 110 is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall 153 is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements 155.

On the working medium side, each heat shield element 155 made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks).

These protective layers may be similar to the turbine blades or vanes, i.e. for example MCrAlX: M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element or hafnium (Hf). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

A for example ceramic thermal barrier coating, consisting for example of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX.

Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).

Other coating processes are conceivable, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may have grains that are porous and/or include micro-cracks or macro-cracks in order to improve the resistance to thermal shocks.

Refurbishment means that after they have been used, protective layers may have to be removed from heat shield elements 155 (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the heat shield element 155 are also repaired. This is followed by recoating of the heat shield elements 155, after which the heat shield elements 155 can be reused.

A cooling system may also be provided for the heat shield elements 155 and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber 110. The heat shield elements 155 are then for example hollow and may also have cooling holes (not shown) which open out into the combustion chamber space 154.

FIG. 5 shows, by way of example, a partial longitudinal section through a gas turbine 100.

In the interior, the gas turbine 100 has a rotor 103 with a shaft 101 which is mounted such that it can rotate about an axis of rotation 102 and is also referred to as the turbine rotor.

An intake housing 104, a compressor 105, a, for example, toroidal combustion chamber 110, in particular an annular combustion chamber, with a plurality of coaxially arranged burners 107, a turbine 108 and the exhaust-gas housing 109 follow one another along the rotor 103.

The annular combustion chamber 110 is in communication with a, for example, annular hot-gas passage 111, where, by way of example, four successive turbine stages 112 form the turbine 108.

Each turbine stage 112 is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium 113, in the hot-gas passage 111 a row of guide vanes 115 is followed by a row 125 formed from rotor blades 120.

The guide vanes 130 are secured to an inner housing 138 of a stator 143, whereas the rotor blades 120 of a row 125 are fitted to the rotor 103 for example by means of a turbine disk 133.

A generator (not shown) is coupled to the rotor 103.

While the gas turbine 100 is operating, the compressor 105 sucks in air 135 through the intake housing 104 and compresses it. The compressed air provided at the turbine-side end of the compressor 105 is passed to the burners 107, where it is mixed with a fuel. The mix is then burnt in the combustion chamber 110, forming the working medium 113. From there, the working medium 113 flows along the hot-gas passage 111 past the guide vanes 130 and the rotor blades 120. The working medium 113 is expanded at the rotor blades 120, transferring its momentum, so that the rotor blades 120 drive the rotor 103 and the latter in turn drives the generator coupled to it.

While the gas turbine 100 is operating, the components which are exposed to the hot working medium 113 are subject to thermal stresses. The guide vanes 130 and rotor blades 120 of the first turbine stage 112, as seen in the direction of flow of the working medium 113, together with the heat shield elements which line the annular combustion chamber 110, are subject to the highest thermal stresses. To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant.

Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). By way of example, iron-based, nickel-based or cobalt-based superalloys are used as material for the components, in particular for the turbine blade or vane 120, 130 and components of the combustion chamber 110. Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blades or vanes 120, 130 may likewise have coatings protecting against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element, or hafnium). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

A thermal barrier coating, consisting for example of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX.

Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).

The guide vane 130 has a guide vane root (not shown here), which faces the inner housing 138 of the turbine 108, and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor 103 and is fixed to a securing ring 140 of the stator 143. 

1. A layer system for application to a surface which is heated to a high temperature, the layer system comprising at least: a substrate on which ceramic layers are applied; at least two ceramic layers on the substrate comprising a bottom layer above the substrate and an outer layer outward of the bottom layer, each layer of the at least two layers is based on zirconium oxide, and each of the at least two layers has respective different properties of the layers; optionally a metallic bonding layer between the substrate and the ceramic layer; the outer layer comprises fully stabilized zirconium oxide; and the bottom layer above the substrate comprises partially stabilized zirconium oxide. 2-3. (canceled)
 4. The layer system as claimed in claim 1, further comprising yttrium oxide as a stabilizer of zirconium oxide in each layer used.
 5. The layer system as claimed in claim 1, further comprising the metallic bonding layer which comprises an MCrAlX alloy, in which X is optionally, at least one of the elements yttrium (Y) or rhenium (Re).
 6. The layer system as claimed in claim 1, wherein each of the outer and bottom ceramic layers has a porosity of at least 16%.
 7. The layer system as claimed in claim 1, wherein the ceramic layers have identical porosities.
 8. The layer system as claimed in claim 1, wherein the outer ceramic layer is at least 20%, thicker than the bottom ceramic layer.
 9. The layer system as claimed in claim 1, wherein the yttrium oxide proportion of the bottom zirconium oxide layer is between 4% by weight and 12% by weight.
 10. The layer system as claimed in claim 1, wherein the yttrium oxide proportion of the outer zirconium oxide layer is between 20% by weight and 50% by weight.
 11. The layer system as claimed in claim 1, wherein the ceramic layers each comprise only zirconium oxide and at least one stabilizer.
 12. The layer system as claimed in claim 1, further comprising only yttrium oxide as a stabilizer of zirconium oxide in each layer.
 13. The layer system as claimed in claim 1, wherein the porosity of each of the outer and bottom ceramic layers is between 16% and 24%.
 14. The layer system as claimed in claim 1, wherein the outer ceramic layer is at least 30% thicker than the bottom ceramic layer.
 15. The layer system as claimed in claim 1, wherein the yttrium oxide proportion of the outer zirconium oxide layer is between 30% by weight and 40% by weight.
 16. The layer system as claimed in claim 1, wherein the bottom layer is a bottommost layer above the substrate; and the outer layer is an outermost layer outward of the bottom layer. 